1. Field of the Invention
The present invention relates to an improved method of steelmaking using an arc furnace. The method realizes, beside increased productivity and decreased energy consumption, a higher yield of refined steel and a smaller amount of waste products.
2. State of the Art
During the process of transition of the type of furnace for steelmaking from open-hearth furnaces to converters, steelmaking using an arc furnace has occupied an important position. This has been supported for the most part by remarkable improvement in productivity of steelmaking in an arc furnace.
When compared with converter steelmaking in which a hot material such as molten pig iron is used, arc furnace steelmaking is handicapped by starting from a cold material such as scrap iron. Accordingly, improvement in the productivity of the arc furnace steelmaking depends on how it could be possible to decrease time and energy for melting the charged cold materials.
One of the main solutions proposed for this purpose is co-called "UHP" electric furnace operation, which uses increased electric power input per unit period of time; and the other is utilization of an auxiliary burner which uses an energy source other than electricity. It can be said that these technologies proved successful to some extent. However, they have limitation in application due to limited capacity of the electric transformer or the burner.
In order to overcome this limitation, there has been tried utilization of oxygen gas. In that technology, during and at the last stage of melting the charged materials with the heat of electric arc, oxygen is blown into the furnace in the maximum amount which the equipment permits; and after formation of smooth molten steel surface, while feeding electric current, oxygen is introduced in an amount excess to the amount necessary for decarburization. In an equipment having an auxiliary burner, oxygen is fed through the burner in an amount excess to the amount necessary for burning the fuel oil. I named the operation of these types "oxygen-enriched operation" in arc furnace steelmaking.
According to the oxygen-enriched operation, melting of solid materials and temperature increase of the molten steel are accelerated by the heat of reaction generated by oxidation of some components in the charged materials, such as carbon, silicon and iron, with the introduced oxygen. Moreover, unmelted large solid pieces adhered to the furnace wall could be cut down by heat to diminish unhomogeneous melting in the furnace. Thus, the productivity of the steelmaking is remarkably improved.
The advantages of the oxygen-enriched operation include not only reduction of the operation period, but also considerable reduction of electric power consumed in the arc furnace for the melting and refining. Intake of heat by arc takes place at the upper part of the charged materials or the molten steel because of the structure of the furnace, and therefore, it is more or less inevitable that a portion of the heat generated is lost due to reflection of radiated heat. On the other hand, the heat of the oxidation reactions occurs at the surface of the charged materials and in the molten steel, and therefore, the most part of the heat can be utilized. Thus, when the efficiency of the total energy input is considered by calculating the used oxygen gas in terms of energy to sum up with the electric energy, it will be higher as the amount of oxygen input increases. (For example, oxygen input of 10 Nm.sup.3 per charged ton will give 10% or more improvement of the efficiency, and at 20 Nm.sup.3, the improvement will reach 20%.)
Although the oxygen-enriched operation is an advantageous method of arc furnace steelmaking as described above, in the process of further seeking the profit thereof, I experienced novel difficulties. One of them is lowered melting yield or the percentage of obtainable molten steel by weight of the charged materials such as scrap iron. The other is, in close relation to the lowered melting yield, increased amount of slag which is residual matter of the melting.